In various situations it is important to verify the authenticity of a photograph—in other words, to verify that a photograph is of a genuine scene which was in front of the lens of a camera at the time the photograph was taken. Examples of these situations include photographs submitted to support insurance claims, and photographs submitted by members of the public to news organisations. Photo editing and re-touching software is now readily available and even relatively cheap consumer-grade editing software can produce edited photographs where the changes are difficult to detect visually.
The applicant's co-pending application Ser. No. 15/863,419, which is incorporated herein by reference, describes a system whereby, by providing trusted software at the point of capture and using cryptographic transformations, the authenticity of a photograph can be to some extent guaranteed, in that with a high level of confidence the system can say that a particular photograph is of the unaltered scene which was in front of a camera at a particular place at a particular time.
However, one way of attempting to circumvent such a system would be to retouch or edit a photograph, to create an artificial or false scene, then to print out the altered photograph, and then to use the “trusted” system of Ser. No. 15/863,419 to take a photograph of the printed photograph at any particular time and place. Alternatively the altered photograph could be displayed on a screen and the screen could be photographed. In the right lighting conditions and with a suitable quality of print or display screen, the result may look visually very real. The assurances given by the trusted system would however not be valid, since the photograph may have been altered, staged in a different location at a different time, or completely falsified.
It is an object of the present invention to detect these “photographs of photographs”.